In recent years, for the purpose of reducing fuel consumption by achieving weight reduction of automobile and ensuring safety for occupants of automobile at the time of a collision, demand for a high strength steel sheet has been increasing. Accordingly, there are growing needs for a high strength of more than conventional 590 MPa to 980 MPa for use in a structural member of automobile, such as a member or a pillar for absorbing energy at the time of a collision. Also, lately, there has been a strong demand for improvement in an antirust property. Therefore, in order to provide both the high strength and antirust property, demand for the galvanized high-strength steel sheet is also growing.
Furthermore, when applying such a steel sheet to an automobile, not only the strength and antirust property but also workability while forming a structural member of the automobile is also an important required characteristic. However, there exists a trade-off between the strength and workability for forming the structural member, and an increase in strength is at the same time accompanied by degradation of workability.
Under the circumstances, in order to improve workability while increasing strength of a steel sheet, a method has been developed such that a composite structure is obtained by transforming austenite into martensite through control of a cooling pattern after heating to a dual phase of ferrite and austenite. Such a composite-structure steel sheet can also be manufactured in a continuous annealing line.
For example, Patent document 1 discloses a method to produce a steel sheet of a composite structure including ferrite and martensite, with which the steel sheet of high workability and high strength is obtained. Moreover, according to Patent document 2, a galvanized steel sheet excellent in terms of strength, anti-aging, and ductility is obtained by specifying the volume rate and grain size of the martensite in the composite structure of ferrite and martensite, a production site and a dispersion state of the martensite, and dispersion intervals.
However, according to Patent document 1, a hot-rolled steel plate is heated at the temperature of 600° C. or more but below Ac1 point and pickling is performed before the recrystallization annealing and tempering treatment are performed, which brings about actual problems of decrease in productivity and increase in cost due to the additional process of heating.
Further, according to Patent document 2, the amount of C contained in a steel material to be used is set to 0.005 to 0.04%. However, when the contained amount of C decreases, martensite for obtaining high strength decreases, so that the strength of 590 MPa or more cannot be obtained. According to Patent document 2, when a large amount of Mo is added as an element for reinforcement, a certain high strength is obtained. However, an increase in material cost is inevitable.
Patent document 1: JP-A No. 2005-213603
Patent document 2: JP-A No. 2005-29867